Secondary alarm filtering

ABSTRACT

A sub-network manager in a telecommunications network such as a synchronous digital hierarchy (SDH) or a synchronous optical network (SONET) filters out secondary (AIS) alarms generated in response to signals received by network elements on incoming links by determining whether the upstream termination point of the incoming link on which the signal was received is within the managed domain of the sub-network manager. If it is within the managed domain, the alarm is blocked. If the upstream termination point is outside the managed domain the alarm is processed further. In the latter case the secondary alarm may be the only indication of the fault causing the alarm that the sub-network manager receives, whereas in the former case it will have received notification from some other alarm, either a primary alarm, such as a loss of signal alarm (LOS) or an AIS alarm generated in response to a signal entering the managed domain from outside.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of European Patent Application No.99302696.2, which was filed on Apr. 07, 1999.

TECHNICAL FIELD

This invention relates to methods of filtering alarm indication signals(AIS) in communications systems such as Synchronous Digital Hierarchy(SDH) and Synchronous Optical Network (SONET) systems.

BACKGROUND OF THE INVENTION

When a fault is discovered in a communications systems such as SDH(synchronous digital hierarchy) or SONET (synchronous optical network)an alarm is generated from the network element detecting the fault,identifying the network element as the source of the alarm, so that thesystem can be reconfigured to avoid routing signals through the affectedlink and/or network element, and so that action can be taken to restorenormal communication. For example, a network element may detect a lossof signal on a link and generate a LOS (loss of signal) alarm. The alarmis passed up to the network element manager.

Since such systems include virtual connections which pass throughseveral physical links (known as a “layer hierarchy”), it is alsonecessary to generate secondary alarms (alarm indication signal (AIS)alarms) from the network elements downstream of the fault. Thus, thenetwork element that detects the loss of signal sends a characteristicsignal (an AIS signal), which consists of a recognisable pattern, suchas a series of ‘1’ bits, along its downstream link to inform downstreamnetwork elements that a loss of signal has been detected. The networkelements that receive such an AIS signal generate AIS alarms which aresent up to the element manager. The network element manager sends thealarms on up to the sub-network manager. The generation of thesesecondary alarm signals means that a single fault can result in a greatmany secondary alarm signals.

An analogy is with a railway network, in which many routes pass throughany one section of track, and each route also passes through manysections of track. Therefore, if one section of track is damaged, thisaffects many stations along the various routes. One damaged section oftrack will therefore give rise to announcements in many stations.

Whilst these secondary (AIS) alarms perform an essential function, theyalso give rise to a problem, in that, from the point of view of thesub-network manager, there are too many alarms raised as a result of asingle fault. The sub-network manager needs to report and keep accountof faults and to maintain a map of the system configuration, but it isnot involved in routing particular messages through the system, so thefewer signals that correspond to a single fault the better; one signalper fault would be ideal. However, the element managers only deal withthe network elements as individual elements, and do not have thehigh-level information to distinguish between signals that thesub-network manager needs and those which it does not need, so they haveto send them all to the sub-network manager. It is not enough simply tosend primary, root cause, alarms, such as LOS alarms, and to withholdsecondary (AIS) alarms, since the source of the primary signal may notbe in the sub-network manager's domain, although the fault affects theoperation of the sub-network manager's domain (i.e. the portion of thenetwork that it controls).

Therefore a method is required which distinguishes between the secondary(AIS) alarms that the sub-network manager needs and those that it doesnot need.

SUMMARY OF THE INVENTION

According to the principles of the invention there is provided a methodof filtering secondary alarm signals sent to a sub-network manager of atelecommunications network wherein the sub-network manager is arrangedto manage network elements in a respective managed domain. Morespecifically, the method comprises determining whether the alarm is asecondary (AIS) alarm generated in response to a signal received by anetwork element on an incoming link. If the alarm is not a secondaryalarm, the alarm is passed for further processing. If the alarm is asecondary alarm, a search for the upstream termination point of theincoming link is carried out to determine whether the upstreamtermination point is in the managed domain of the sub-network manager.If the upstream termination point is in the managed domain, the alarm isblocked. If not in the managed domain, the alarm is passed for furtherprocessing.

Accordingly, since the sub-network manager has information on theconnectivity of the network domain, it can determine whether the networkelement that originates the alarm is on the edge of the managed domain.If the upstream link is terminated by a termination point which is notin the managed domain, the alarm is passed. If it is terminated by atermination point which is in the managed domain the alarm is blocked.Thus, the only secondary alarms that are passed are those that aregenerated as a response to a signal entering the domain from outside.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in simple diagrammatic form, a sub-network in which theinvention could be implemented.

FIG. 2 is a flow diagram, showing how the sub-network manager of FIG. 1filters AIS alarms.

DETAILED DESCRIPTION

FIG. 1 shows a number of network elements, 1.0-1.6, connected to formpart of a telecommunications network such as a SDH or SONET type ofnetwork. The network elements each contain all the functionalityrequired to route incoming signals, but they do not have any informationon the topology of the network, so they have to rely on higher levels ofcontrol to configure them so that they route the signals to the correctdownstream elements.

The network elements 1.1-1.6 are controlled and monitored by an elementmanager 2 which deals with the network elements as individual units.That is to say, it does not have information on the network topology,but it does have information on the individual network elements—whattype of equipment they contain and what exchanges of signals arenecessary to reconfigure them. Although only six network elements areshown connected to the element manager 2, in a typical network theremight be of the order of two hundred network elements controlled by oneelement manager.

The connecting links between the network elements may comprise opticalfibre links which may carry of the order of 16×63=1008 channels. Many ofthese channels will be configured as part of virtual connections. Avirtual connection is a semipermanent arrangement of channels configuredto provide a connection between end-points which are not directlyconnected by the network. In fact, the end points of a virtualconnection may be in different parts of the network, managed bydifferent sub-network managers.

The element manager is connected to a sub-network manager 3 whichcontrols the section of the network in its domain as a set of linksforming end-to-end connections. To do this, it needs to have informationon the topology of the part of the network in its domain and to maintaina map of which links or network elements are non-operative, so that itcan reconfigure the network to avoid them while maintaining the requiredend-to-end connections. Therefore, it needs to receive alarms that areraised by the network elements indicating non-functionality of some partof the network.

Although FIG. 1 shows only one element manager 2 connected to thesub-network manager 3, it is presently contemplated that up to about tenelement managers may be connected to a single sub-network manager, so asingle sub-network manager may have somewhere of the order of 200 to2000 network elements in its management domain.

The sub-network manager 3 includes a processor 4 and a memory 5 arrangedto carry out its functionality including the filtering method of theinvention. Included in the memory 5 is a table 6 which includesinformation about the links connected to the network elements 1.1-1.6 inthe managed domain.

The sub-network manager is in turn connected to a higher-level manager(not shown), culminating in a manager for the whole network.

If the link running from network element 1.1 to network element 1.3 isdamaged and becomes non-functional, the downstream network element 1.3detects a loss of signal and sends a LOS alarm up to the element manager2. Since alarms such as this affect the links that are available to thenetwork, the element manager 2 passes them up to the sub-network manager3, which needs to maintain its map of functional and non-functionallinks. Since the affected link may be part of many virtual connections,other network elements than 1.3 are affected by the damage to the link,and the network element 1.3 generates an AIS signal (e.g. a string of‘1’ digits or some other recognisable signal) and sends it out on thedownstream links. The downstream network elements that receive the AISsignal send an AIS alarm to their element manager, which may not be thesame as the element manager 2 of the element 1.3 that sent the LOSalarm. The element manager passes the AIS alarm to the sub-networkmanager, which may not be the same sub-network manager 3 as the one thatreceived the original LOS alarm.

The alarms contain information identifying what type of alarm they are(e.g. LOS or AIS) and which termination point generated the alarm. Thus,in the case of the original LOS alarm, the alarm will identify itself asa LOS alarm and identify the termination point of the link from element1.1 to element 1.3. This will provide information that the sub-networkmanager 3 needs to maintain its map, namely that the particular linkthat is identified has become non-functional. Once the sub-networkmanager 3 has received this information, it has all it needs to dealwith the problem. The AIS alarms generated by the downstream elements donot provide anything new. On the other hand, it may be that an AIS alarmis the first or only indication of a problem occurring upstream, outsidethe domain of the sub-network manager 3. Since the element managercannot determine whether or not this is the case, it has to pass all AISalarms up, with the result that the sub-network manager will receivemany, possibly a great many, AIS alarms every time a problem arises inits domain, or, in some cases, outside it.

FIG. 2 shows the method carried out by the processor 4 by which thesub-network manager filters incoming alarms so as to eliminateirrelevant AIS alarms.

When an alarm is received 10 it is examined to determine whether it isan AIS alarm 11. If it is not, then the alarm is passed 15, since it maycontain relevant information. If the alarm is an AIS alarm it isexamined to determine which termination point raised the alarm, and thetable of links 6 held by the sub-network manager is searched 12 for theupstream termination point of that link to determine whether it is inthe managed domain of the sub-network manager 13. If it is not in themanaged domain (for example, the upstream termination point is not inthe table and the search 12 fails), the alarm is passed 15, whereas ifit is in the managed domain the alarm is blocked 14.

In the case we have been considering, the damaged link is that runningfrom element 1.1 to element 1.3. Element 1.3 raises a LOS alarm which ispassed up to the sub-network manager 3. This LOS alarm identifies itselfas a LOS alarm and identifies the termination point that detected theloss of signal, i.e. the termination of the link from element 1.1 to1.3. It also sends an AIS signal downstream. When a downstream element,say 1.4, detects the AIS, it generates an AIS alarm, which is also sentup to the sub-network manager 3. The AIS alarm identifies itself as anAIS alarm and identifies the termination point which received the AISsignal.

When the sub-network manager 3 receives the LOS alarm, the alarm ispassed 15, since it is not an AIS alarm, and the sub-network managerprocesses it further, using the information contained in it to identifythe problem that caused it.

When the sub-network manager 3 receives the AIS alarm from element 1.4the processor 4 identifies it as an AIS alarm 11, determines 12 that itrelates to the link from element 1.3 to 1.4 and looks up that link inits table 6. Thus it determines 13 that the upstream link terminationpoint 1.3 is in the managed domain, so it blocks the alarm 14 and doesnot process it any further. Since it was caused by same problem as theLOS alarm, the AIS alarm would not have provided any additionalinformation.

On the other hand, if a network element, such as element 1.0, which isnot in the managed domain of sub-network manager 3, detects a loss ofsignal or an AIS signal on a link other that that from element 1.1, itwill send an AIS signal downstream to network element 1.1, whichgenerates an AIS alarm identifying itself as an AIS alarm andidentifying the termination point which received the AIS signal.

When the sub-network manager 3 receives this AIS signal and theprocessor 4 comes to look up the link data in the table 6, it finds thatthe upstream link termination point 1.0 is outside the managed domainand therefore passes the alarm. In this case, the alarm was caused by aproblem occurring outside the managed domain, so the sub-network manager3 will not have been informed of the problem, though it affects theoperation of the managed domain. Therefore the processor needs toprocess the alarm further.

Element 1.1 also sends an AIS signal downstream to element 1.3, whichalso generates an AIS alarm, but this time, the processor 4, when itlooks up the link data, finds that the upstream termination point 1.1 isin the managed domain, so it blocks the alarm and does not process itfurther. The alarm does not contain any information not alreadycontained in the AIS alarm from element 1.1, so it is not necessary forthe processor to process it further.

Thus, the method illustrated in FIG. 2 effectively frees the sub-networkmanager from being overloaded with information which is not relevant toit. It does this merely by looking up the link data for the alarm, sothe filtering process itself does not impose any great computationalburden on the sub-network manager.

The foregoing is merely illustrative of the principles of the invention.Those skilled in the art will be able to devise numerous arrangementswhich, although not explicitly shown or described herein, neverthelessembody those principles that are within the spirit and scope of theinvention. Accordingly, the invention is only limited by the claims thatfollow.

What is claimed is:
 1. A method of filtering alarms sent to asub-network manager of a telecommunications network, the sub-networkmanager being arranged to manage network elements in a respectivemanaged domain, the method comprising: determining whether the alarm isa secondary (AIS) alarm, generated in response to a signal received by anetwork element on an incoming link; if the alarm is not a secondaryalarm, passing the alarm for further processing; if the alarm is asecondary alarm, searching for the upstream termination point of theincoming link and determining whether the upstream termination point isin the managed domain of the sub-network manager; if the upstreamtermination point is in the managed domain, blocking the alarm; and ifthe upstream termination point is not in the managed domain, passing thealarm for further processing.
 2. A method according to claim 1 whereinthe method is carried out within the sub-network manager.
 3. A methodaccording to claim 2 wherein a secondary (AIS) signal includes dataidentifying the link termination point that received the signal inresponse to which the alarm was generated, and the step of searching forthe upstream termination point of the incoming link comprises using thedata identifying the link termination point to retrieve link data from atable held by the sub-network manager.
 4. A method according to claim 1wherein the network is a synchronous digital hierarchy (SDH) network. 5.A method according to claim 1 wherein the network is a synchronousoptical network (SONET).
 6. A method of filtering alarms sent to asub-network manager of a telecommunications network, the saidsub-network manager being arranged to manage network elements in arespective managed domain, the method comprising: determining whetherthe alarm is a secondary (AIS) alarm, generated in response to a signalreceived by a network element on an incoming link; if the alarm is not asecondary alarm, passing the alarm for further processing; if the alarmis a secondary alarm, searching for the upstream termination point ofthe said incoming link and determining whether the said upstreamtermination point is in the managed domain of the sub-network manager;if the upstream termination point is in the managed domain, blocking thealarm; and if the upstream termination point is not in the manageddomain, passing the alarm for further processing; wherein the networkincludes at least one virtual connection having a termination pointwithin the managed domain and a termination point not in the manageddomain.
 7. A sub-network manager for a telecommunications network, thesub-network manager being arranged to manage network elements in arespective managed domain, the sub-network manager comprising: means fordetermining whether the alarm is a secondary (AIS) alarm, generated inresponse to a signal received by a network element on an incoming link;means for passing the alarm for further processing if the alarm is not asecondary alarm; means for searching for the upstream termination pointof the incoming link and determining whether the upstream terminationpoint is in the managed domain of the sub-network manager if the alarmis a secondary alarm; means for blocking the alarm if the upstreamtermination point is in the managed domain; and means for passing thealarm for further processing if the upstream termination point is not inthe managed domain.
 8. A telecommunications network including asub-network manager being arranged to manage network elements in arespective managed domain, the sub-network manager comprising: means fordetermining whether the alarm is a secondary (AIS) alarm, generated inresponse to a signal received by a network element on an incoming link;means for passing the alarm for further processing if the alarm is not asecondary alarm; means for searching for the upstream termination pointof the incoming link and determining whether the upstream terminationpoint is in the managed domain of the sub-network manager if the alarmis a secondary alarm; means for blocking the alarm if the upstreamtermination point is in the managed domain; and means for passing thealarm for further processing if the upstream termination point is not inthe managed domain.